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(************************************************************************)
(* v * The Coq Proof Assistant / The Coq Development Team *)
(* <O___,, * CNRS-Ecole Polytechnique-INRIA Futurs-Universite Paris Sud *)
(* \VV/ **************************************************************)
(* // * This file is distributed under the terms of the *)
(* * GNU Lesser General Public License Version 2.1 *)
(************************************************************************)
(*i $Id$ i*)
(** This module proves the validity of
- well-founded recursion (also called course of values)
- well-founded induction
from a well-founded ordering on a given set *)
Set Implicit Arguments.
Require Import Notations.
Require Import Logic.
Require Import Datatypes.
(** Well-founded induction principle on [Prop] *)
Section Well_founded.
Variable A : Set.
Variable R : A -> A -> Prop.
(** The accessibility predicate is defined to be non-informative *)
Inductive Acc (x: A) : Prop :=
Acc_intro : (forall y:A, R y x -> Acc y) -> Acc x.
Lemma Acc_inv : forall x:A, Acc x -> forall y:A, R y x -> Acc y.
destruct 1; trivial.
Defined.
(** Informative elimination :
[let Acc_rec F = let rec wf x = F x wf in wf] *)
Section AccRecType.
Variable P : A -> Type.
Variable
F :
forall x:A,
(forall y:A, R y x -> Acc y) -> (forall y:A, R y x -> P y) -> P x.
Fixpoint Acc_rect (x:A) (a:Acc x) {struct a} : P x :=
F (Acc_inv a) (fun (y:A) (h:R y x) => Acc_rect (x:=y) (Acc_inv a h)).
End AccRecType.
Definition Acc_rec (P:A -> Set) := Acc_rect P.
(** A simplified version of [Acc_rect] *)
Section AccIter.
Variable P : A -> Type.
Variable F : forall x:A, (forall y:A, R y x -> P y) -> P x.
Fixpoint Acc_iter (x:A) (a:Acc x) {struct a} : P x :=
F (fun (y:A) (h:R y x) => Acc_iter (x:=y) (Acc_inv a h)).
End AccIter.
(** A relation is well-founded if every element is accessible *)
Definition well_founded := forall a:A, Acc a.
(** Well-founded induction on [Set] and [Prop] *)
Hypothesis Rwf : well_founded.
Theorem well_founded_induction_type :
forall P:A -> Type,
(forall x:A, (forall y:A, R y x -> P y) -> P x) -> forall a:A, P a.
Proof.
intros; apply (Acc_iter P); auto.
Defined.
Theorem well_founded_induction :
forall P:A -> Set,
(forall x:A, (forall y:A, R y x -> P y) -> P x) -> forall a:A, P a.
Proof.
exact (fun P:A -> Set => well_founded_induction_type P).
Defined.
Theorem well_founded_ind :
forall P:A -> Prop,
(forall x:A, (forall y:A, R y x -> P y) -> P x) -> forall a:A, P a.
Proof.
exact (fun P:A -> Prop => well_founded_induction_type P).
Defined.
(** Building fixpoints *)
Section FixPoint.
Variable P : A -> Type.
Variable F : forall x:A, (forall y:A, R y x -> P y) -> P x.
Fixpoint Fix_F (x:A) (r:Acc x) {struct r} : P x :=
F (fun (y:A) (p:R y x) => Fix_F (x:=y) (Acc_inv r p)).
Definition Fix (x:A) := Fix_F (Rwf x).
(** Proof that [well_founded_induction] satisfies the fixpoint equation.
It requires an extra property of the functional *)
Hypothesis
F_ext :
forall (x:A) (f g:forall y:A, R y x -> P y),
(forall (y:A) (p:R y x), f y p = g y p) -> F f = F g.
Scheme Acc_inv_dep := Induction for Acc Sort Prop.
Lemma Fix_F_eq :
forall (x:A) (r:Acc x),
F (fun (y:A) (p:R y x) => Fix_F (Acc_inv r p)) = Fix_F r.
destruct r using Acc_inv_dep; auto.
Qed.
Lemma Fix_F_inv : forall (x:A) (r s:Acc x), Fix_F r = Fix_F s.
intro x; induction (Rwf x); intros.
rewrite <- (Fix_F_eq r); rewrite <- (Fix_F_eq s); intros.
apply F_ext; auto.
Qed.
Lemma Fix_eq : forall x:A, Fix x = F (fun (y:A) (p:R y x) => Fix y).
intro x; unfold Fix in |- *.
rewrite <- (Fix_F_eq (x:=x)).
apply F_ext; intros.
apply Fix_F_inv.
Qed.
End FixPoint.
End Well_founded.
(** A recursor over pairs *)
Section Well_founded_2.
Variables A B : Set.
Variable R : A * B -> A * B -> Prop.
Variable P : A -> B -> Type.
Variable
F :
forall (x:A) (x':B),
(forall (y:A) (y':B), R (y, y') (x, x') -> P y y') -> P x x'.
Fixpoint Acc_iter_2 (x:A) (x':B) (a:Acc R (x, x')) {struct a} :
P x x' :=
F
(fun (y:A) (y':B) (h:R (y, y') (x, x')) =>
Acc_iter_2 (x:=y) (x':=y') (Acc_inv a (y, y') h)).
Hypothesis Rwf : well_founded R.
Theorem well_founded_induction_type_2 :
(forall (x:A) (x':B),
(forall (y:A) (y':B), R (y, y') (x, x') -> P y y') -> P x x') ->
forall (a:A) (b:B), P a b.
Proof.
intros; apply Acc_iter_2; auto.
Defined.
End Well_founded_2.
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